var config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
        int initialCount = ConfigurationManager.ConnectionStrings.Count;
        string connStringName = "TEST";
        string serverName="Servedr";
        string databaseName = "MyDb";
        string userId="MyUseId";
        string password="MyPassword";
        var connectionStringBuilder = new SqlConnectionStringBuilder
                                          {
                                              DataSource = serverName,
                                              InitialCatalog = databaseName,
                                              UserID = userId,
                                              Password = password
                                          };


        var csSetting = new ConnectionStringSettings(connStringName, connectionStringBuilder.ConnectionString, "System.Data.SqlClient");
        var csSection = config.ConnectionStrings;
        csSection.ConnectionStrings.Add(csSetting);
        config.Save(ConfigurationSaveMode.Modified, true);
        ConfigurationManager.RefreshSection("ConnectionStrings");

        int finalCount = ConfigurationManager.ConnectionStrings.Count;

var csSetting = new ConnectionStringSettings(connStringName, connectionStringBuilder.ConnectionString, "System.Data.SqlClient");

        var readonlyField = typeof(ConfigurationElementCollection).GetField("bReadOnly", BindingFlags.NonPublic | BindingFlags.Instance);
        readonlyField.SetValue(ConfigurationManager.ConnectionStrings, false);

        var baseAddMethod = typeof(ConfigurationElementCollection).GetMethod("BaseAdd",
            BindingFlags.Instance | BindingFlags.NonPublic, null, new Type[] { typeof(ConfigurationElement) }, null);
        baseAddMethod.Invoke(ConfigurationManager.ConnectionStrings, new object[] { csSetting });

        readonlyField.SetValue(ConfigurationManager.ConnectionStrings, true);

        int finalCount = ConfigurationManager.ConnectionStrings.Count;